The army of north Sudan is demobilizing some 15,000 southerners from its ranks, as the south gets set to declare independence.

The pro-government Sudanese Media Center reports there will be a dismissal ceremony in Khartoum Thursday, to be addressed by Defense Minister Abdel Rahim Mohammed Hussain.

The new Republic of South Sudan is due to declare independence on Saturday. The north and south are trying to disentangle themselves but the sides have yet to resolve issues on borders and oil revenue.

The north's army is currently fighting pro-southern elements in the northern-controlled state of Southern Kordofan.

Meanwhile, rehearsals are underway for Saturday's celebrations in the south's capital, Juba. Authorities have closed streets so organizers can prepare for the main independence ceremony and other festivities.

Some 30 African heads of state are expected to attend, including Sudanese President Omar al-Bashir and U.N. Secretary-General Ban Ki-moon.

Mr. Ban called Thursday for a smooth secession and an immediate ends to clashes in Southern Kordofan.

Also Thursday, the U.S. ambassador to the United Nations, Susan Rice, said Saturday's ceremonies marking the independence of South Sudan are historic events, but also a fragile moment. She said the situation remains tense and that the northern and southern governments must resolve issues related to their border areas. Rice said allowing the disputes to linger could cause social upheaval and economic shocks and destabilize the future relationship between the two states.

North and south Sudan fought a 21-year civil war that ended in 2005. Southern Sudan voted overwhelmingly to split from the north in a January referendum.